Note: Descriptions are shown in the official language in which they were submitted.
2 ~
- 1 ~ 55, 19
-- APPARATUS AND M~l~OD FOR :I~PROVING FII~
l~RAP~PIT 0~ A ~ISTUR~ PRl~-S~ TOR
FOR A 5TEAM l~BINlS
BA~ICGR013ND OF T~ INV~iTIQN
SField o~ the Invention:
The present invont~on xelates generally to film
entrapment moisture pre-se~arators ~or control or
eliminatlon of nuclsAr high preæsur0 turblne exhaust
piping wall deterioratlon which is assoclated with the
10erosion-corrosion phenome~on.
Descri~tion o~ the Rel~ted _ :
The w~t steam conditions associated with a nuclear
steam turbine cycle have been observed to cause
signi~icdnt eros~on/corrosion of cycle steam piping and
componen~s between the high pressure turbine exhaus~
and the moisture separa~or reheater~ The pattern,
location and extent of cross-under piping
erosion/corrosion ls a function of pip~ng size,
- 2 - 55,198
ma~erial and layout configuration, turbine exhaust
conditions and plant load cycle. However, as a general
rule, a base~loaded plant having car~on steel cross-
under piping with typical nu~lear high pressure turbine
exhaust conditions of 12% moisture and 200 psia will
experience~ within 3 to 5 years aftrr initial start up,
erosion/corrosion dama~e levels that require weld
repair to restore minimum wall thickness~ Such weld
repalrs are expensive, time consuming and of~en result
in extended planned outages and occasional unscheduled
_ outages.
Weld repair of erosion/corrosion in cross-under
piping is expensive~ but ~he alternative o~ partial or
complete replacement of the eroded pip~ng is even more
expensive, ~ven the time reguirements and logistics
lnvolved ln plpe replacement.
The bulk of piplng wa:Ll erosion in nuclear plants
has been found to be the result of a ~orm of metal
attack referred to as l'~low as~isted corrosion" ~FAC).
FAC-type erosion ~an oc~ur anywhere in a piping system
where a high purity wa~er film at~aches to and moves
- over A surface. Under the ~emperature ran~e normally
associated with nuelear power plant hi~h pressur~
~urbine exhaust piping (250F to ~50F) these hlgh
purity water films ha~e the ability to dissolve the
normally protective magnetit~ layer in such a manner
that ~ontinuous oxidation o~ the steel below the
magnetite layer will occur. FAC-type corrosion
mani~ests itself in piping systems as scalloped~out or
flu~ed regions, which are indicative o~ maæs transfer
occurrence as a result of the magnetite dissolution.
, . . .
2 ~ J2
- 3 - 55,198
The necessary water film associated with FAC-type
erosion/corrosion is~ in the case o~ high pressure
nuclear ~urbine exhaust piping, ~rea~ed by ~he high
pressure turbin~. By virtue of its yeometry, a nuclear
high pressure steam ~ur~ine exhaust casing creates
vortices in the exitin~ wet steam. Such vortices have
long been observed ln curved piping where they are
known as secondary ~low patterns. The two phase ~low
in a curved conduit is described in U.S. Patent
4,803,845 and ls illustrated in Figs. 1 and 2.
_ Basically, nuclear turbine exhaust casings create
vortices and genera~e a centrifugal force field causin~
it ~o function as a centri~u~al separator by forcing
the heavier or larger water droplets to migrate or
dxift through the gas phase (steam) and ~e deposited on
the exhaust casing wall. The extent of separation
depends on the steam flow or veloci~y, exhaust casing
geometxy ~primarlly the radius of curvature), and steam
condition such as pressure, temper~ture and guality.
~y csnsid2ring the resul~ing centrifugal force and the
resisting drag force under typical exhaustlng
- Gonditions, the relative vlelocity of moisture droplets
50 ~m or bigger with respect to the steam will result
in tra~ectories such that 20 to 30% of the total
moisture pressnt at the exi~ of the high pressure
turbin~ could appear as a water film on th~ exhau~t
casing w~lls. This hi~h purity wate~ film is
apparently responsible for the FAC observed in the
down~tream exhaust and extractlon ~iping. It has been,
therefore, a long standing problem to remove this film
- 4 - 55,19
before i~ can pass into the outlet nozzle in order to
reduce or eliminate cross-under piping FACo
Since moisture separators are already presen~ as
an in~erstage element between the high pressure turbine
steam exhaust and the low pressure turbine inlet, the
devi~es to remove moisture in the steam be~ore it
enters the existing separators are known a~ moistuEe
pre-separator or simply "pre-separators".
Specifically, pre-sep~rators that interrupt the water
film prior to its entrance into the exhaus~ piping
_ proper are referred to s "in-turbine film entrapment"
~ype pre-separators.
One type of in~turbine film entrapm2nt pre-
separator is illustrated in Fig. 3. The pre-separator
"sk:Lms" the water film o~f the turbine exhaust caslng
wal:ls in the exhaust nozzle- exhaust casing interface
region and collect~ the water in a small annular
chamber between th~ s~imm~r body and the exhaust
nozzle. This chamber acts~ as a moisture collection
cavity, but provide~ little hold-up volume and thus
requlres the drillin~ of s,ome (typically four) large
drain llnes (larger relati.v~ to the collection ~hamber
volume) through the turbine nozzle-casins walls.
Th~ in-line pre-separator illustrated ln Fig. 4 is
describ~d in det~il in U.S. Patent No. 4,803,841. The
pre~separator described therein prov~des a struc~ure
having a condensate collection zone located outslde the
turbine proper as a ~acketed cross-under pipe. The
dimensions an~ con~iguration of the coll~ction chamber
are varied as dcsired and multiple draln lines are
located around the periphery of thè coll~ction chamber,
- S - 55,198
not necessarily in uniform spacing, to best suit
backfit situations and thus minimizing the need to
relocate existing piping and avoiding expensive
modification or in~erferences with existin~ structures.
Basically, the pre-separator lncludes a pre-separator
body formed around an existing cross-u~der piping to
form an annular moisture collection ehamber. Flow
director plates are u~ed to channel the water film flow
(as indicated by directional arrows) into the annular
collection chamber. This pre-separator is described in
_ ~reater detail in the aforementioned U.S. patent. The
upper ex~ension cylinder geome~ry is normally tailored
to provide a controlled entry gap between the exhaust
.casing inner diameter walls and the leading edge of ~he
upper extension cylinder. The upper extension cylinder
thus provides the skimmer function of the in-line film
entrapment type pre-separator.
The prime requisite ~or a properly ~u~ctioning
entrapment pre-separator ,regardless of speclfic
applicàtion is t~ contro11ed tnarrow~ opening or gap
provided by the upper extension cyllnder 1%ading edge
- and the high pressure turbine exhaust casing in the
intersection region betwe~n the turblne exhaust ca~ing
volute and exhaust nozzle opening. It is this design
requirement that is the most diffi~ult to achiave and
has been a ~ause for reduction in water film capture
efficiency. For ëxample, in the configuration
illustrated in Fig. 4, lt has been estimated based on
prior experience that between 20 and 35~ of the total
moisture in a particular high pressure turbine exhaust
would be on or very near the exhaust casing volute
- 6 ~ 55,198
walls and could be trapped by the skimmer- Once placed
in service, however, pre-separ~tor performance has
indicated a lower percent of the total moistur2 being
collected than expected. It was discovered that one
source of the problem was the exhaust steam velocity
around the periphery of the h.p. turbine exit nozzles
and the pre-separator upper extension cylinder ~where
the water film o~ entrance is located) was very non-
uniform. This discovery was made in a care~ully
instrumented test series using a one sixth scale model
_ air-water facility. These scale model tests were
carefully designed to replicate high pressure turbine
exhaust chamber velocities, phase separation
characteris~ics between liquid and gas (film production
on the exhaust casing walls) and hydraulics in the pre-
separator and turbine exhaust. As is kr.own in the
field o~ hydraul~cs, this non-uniform approach velocity
will ~et up a non-uniform pressure ~ield or pressure
gradient around the entry gap. Such pressure yradients
then elther locally block entry of the water ~ilm into
the annular gap and/or cause the wa~er ~ilm captured to
~e swept out of the annulus back into the main ~low,
thus bypassing the pre-separator collection chamber.
Although rsducing the pre-separator ent~y gap offers
some minor ~mprovement in water film capture by
increasing the pressure drop through the entry gap,
such an approach is lmpractical in practice and is not
suf~iciently effective ~o of~se~ totally the large
pr~ssure gradient observed in the scale model flow
test.
_ 7 ~ 55,198
One method to overcome this loss of water film
capture resulting from a pressure gradient or pressure
recovery occurrin~ around the en~ry gap is ~o provide a
mo~ive fluid usin~ the carrier gas ox vapor to entrain
the captured fluid film. Such concepts have been used
in moisture extrac~ion zones of steam turbines and has
formed the basis for one type of pre-separator
discuss~d in U.S. Patent No~ 4,624,111. This motive
fluid approach in realtty provides 2 ven~lng mechanism
for relievlng the pressure build up that would occur if
_ the velocity of the carrier ga~ entraining the
moisture, or in this sl~uatlon, dragging a water film
along the turbine exhaust casing walls, is brought to a
near stagnation Izero veloci~y~ condition. In past
practice, however, th@ standard ven~ing scheme has
involved large external piping sys~ems. In tuxn, the
large amount of motive fluid ~steam) necessary mus~ be
separated from the entrainled water fllm, for reasons
for economy, and th~ motivle fluid returned into the
system. Thi~ reguires exp~ensive and complicating
features, especially so wh~eh applied to installing a
~ pre-separator ~nto an existin~ nuclear steam turbine
system.
SUMMARY OF T~E I~V~TION
An ob~ect of the pre~nt invention is to provide
an apparatus and method which uses a motive flu~d ~or
enhancing fluid fllm capture in a film entrapment pre-
separator without the n~ed for large expensive external
piping and phase separation equipment to process motive
steam as required in all previous art~
- 8 - 55,198
Another object of the present invention is to
provide a pre-separator ~or a steam ~urbine which vents
an annular collection chamber internally, without
reguiring external piping o~ motive 1uid.
Another object of the present invention is to
provide a pre-separator ~or a ste~m turbine which is
relatively simple in construction and cost-effe~tive to
produ~e.
Another object of the present invention is to
equ~lize pressure around an entry gap of a pre-
separator so as to increase film entrapment
effectiveness.
In a preferred embodimen of the present
invention, a moisture pre~separator for a steam turbine
havi.n~ ~n exhaust portion o~ an exhaust nozzle include~
an inner cylinder having inner and outer sur~aces, an
outer cyl~nder having inner and outer surfaces and
being concentric with the inner cylinder, a bottom
interconnecting lower ends o~ ~he inner and outer
cylinders, an annular coll~ction chamber formed by the
outer surface o~ th~ inner cylinder, the inner surface
o~ the outer cylinder, and, the bottom, drains disposed
in the outex cylinder near the bo~tom for draining
moisture collected in th~ annular collection chamber,
2S an upper cylinder extension connected to and extending
the inner cylind~r into the exhaust nozzle of the
exhaust portion o~ the ste~m tur~ine, an entry gap
being ~ormed ~etween t~e upper cylinder extension and
the exhaust nozzle, the inner and outer cylinders and
bottom forming a pre-separator body which is
connectable at one end to cro~s-under piping ~nd to the
~$ ~ ?~
- 9 - 55,198
nozzle at the other end so as to communicate steam
~hrough an lnterior of the inner cylinder, and vent
means for co~municating steam from the collection
. chamber to the interior of the inner cylinder, thereby
equalizing pressuxc around the circumference of the
entry gap and increasing effec~lveness of moisture
entrapment.
The foregoing and other fea~ures and advantages of
the pre-separator in accordance with the present
invention will becom~ more apparent from the following
_ detailed description, taken in conjunction with the
drawings.
BRIFF DRSCRIPTION OF TE~ DRA~INGS
Fi~. 1 is a side elevational view, partly in
section, showing helical flow patterns through a bend
portion of a pipe;
Fig. 2 is a sectlonal view t~ken along line II-II
of Fig. l;
Fig. 3 is a sectional view illustrating a known
pre-separator having a skin~er body disposed in an
- ~xhaust no~zle portion of a turbine casing;
Fig. 4 is a sid~ elevatlonal view, partly in
section, showing Xnown pre~eparators;
Flg. S ~s a s~ctional view of a pre-separator
according to one embodlment of the present invention;
Fig. 6 is a schematic, cross~sectional view of the
pr~-separator of Fig. 5, illustrating center lines of
vent holes;
~J~ P~t'~
- 10 - 55,198
Fig. 7 is a cross-sectional view of the pre-
separator of Fig. 5, illustrating vanes disposed on
opposite side walls Qf the collection chamber;
Fig. 8 is an enlarged, sectional view of the pre-
separa~or of Fig. 5; and
Fig. 9 is a side elevational view of a portion of
the inner cylinder, illustrating a vent hole and weir.
DETAIL~ DRSCRIPTION OF ~H~ P~ZF~RRED
EMBODIMENTS
Referring to Fig. 4, an in-line pre-separator used
in an exhaust portion 21 of a ste~m turbin~ is
generally re~erred to by the numeral 20 and includes a
pre separator body 22 and an upper extension cylinder
24. The pre-separator body 22 is formed by two
concentric cylinders, the outer cylinder 26 being
join~d to the high pressure turbine exhaust noæzle 34
to ~orm a pressure boundary. The inner cylinder 28
replaces a removad section of cross-under p~pe ~. The
inner and outer cylinders ~6 and 28 are ~o~ned at their
lower ends to a bottom 31 to form an annular collection
_ chamber 30 which receives the moisture separated from
the steam. Drains 32, not: necessarily uniformly spaced
around the pre-sep~ratox body outer cixcu~ference,
prov~des means to drain collected moisture from the
annular collection chamber 30.
The upper extension cylinder 24, haviny an outer
di~meter vexy closely matching that o the inner
diameter of the pre-separator body inner cylinder 28,
is slidably positioned within and th~n ~oined to the
pre-sepaxator body inner cylinder 28, such that the
?
~ 55,198
leadlng edge of the upper end of the upper extension
cylinder 24 forms a n~rrow gap 25 between the inside
surface of the turbine exhaust casing-nozzle 34 and the
upper extension cylinder 24 outer sur~ace. TAiS gap or
opening, although variable in size around the periphery
of ~he ~urbine exhaust casing-exhaust nozzle re~ion,
forms a generally narrow annular condu~t for passage of
~he water film skimmed from the turbine wall down into
the annular collection ~h~mber of the pre-separator
body.
_ Although this annular conduit is sufficiently
larqe to easily pass the thin water film flowing around
the turbine exhaust casing, the high ~elocity c~rrier
steam also enters the gap and is brought to near
' stagnation conditions in the constric~ed flow pa~h.
Because both the annular gap flow cross-section and the
entering steam velocities are non-uniform around the
per1.phery o4 the annular gap, the conversion of steam
momentum into pressure varies around the periphery of
the annular opening. This re~ults in a pressure
gradient around the annular gap periphery that locally
prevents the ma~ority of the water from enter~ng the
gap and/or r~directs the film from continuing its
swirling flow down into the annular gap towards the
pre-separa~o~ body by forcing it to follow paths
gen2rally parall21 to the annular opening towards zones
o~ lower pressure where the ~low direction and
acceleration cause it to be forced out of the gap and
returned to the main body of the steam flow. The
overall action of this periphexal pressure gradisnt
around the annular gap is to create short circuit paths
- 12 - 55,198
in which the water film is first forced around rather
than down the pre-separator upper extension cylinder 24
and 'hen back into the cross-under line, thus reducing
pre-separator moisture removal effectiveness.
~y virtue of the hydraulic principles discussed
above, the annular collection ohamber 30 in the pre-
separator body 22 is positively pressurized with
respect to the main steam flowing in the circular
cross-section of the ~ nner cylinder 28. Thi~ positive
pressure is a direc~ consequence of the pressure build
_ up in the entry gap 25 around the upper extension
cylinder 24.
It is known to provide venting or prescure relief
by connecting the pre~separator annular moisture
coll~ction volume to a l~wer pressure source in order
to counteract or eliminate this undesirable pressure
gradient. For example, one method lnvolves venting the
collection volume to a lower pressure external to the
entire pre-separator propex ~meaning that the lower
pressure is.outside the erltire exhaust system piping).
The present invention provides another method to
- relieve this pressurization of the annular collection
chamber by provlding the necessary r~lief venting using
properly arranged and sized vent holes through the pre-
separato~ body inner cylinder wall, thereby dlrec~ly
connecting the pre-separator body annular moisture
collection chamber 30 with the turbine exhaust s~eam
flow indicated by direational arrow "~" in Fig. 4.
This venting arrangemen~ allows the carrier gas (steam)
entering the pre-separator with the moisture film to
. flow down into the pre-separator annular collection
d 2
- 13 - 55,198
chamber in order to substantially reduce the pressure
gradient around the upper extension cylinder-turbine
casing entry ~ap 25. This permits the moisture film on
the ~urbine caslng wall to flow past the entry gap 25
and on into the pre-separator annular collection
chamber 30. This results in an internal venting of the
motive fluid (steam).
Figs. S and 6 illustrate a first, preferred
embodiment of the invention. The pre-separa~or 20 has
a pre-separator body 22 which is formed by an outer
_ cylinder 26 having inner and outer surfaces and an
inner cylinder 28 having lnner and ou~er surfaces. An
annular collection chamber 30 is formed between the two
cylinders 26, 28 and a bottom 31 and is provided with
drains 32 in a lower portion thereof of ~he outer
cylinder 26 near the bottom. The pre-separa~or 20 has
the same basic structure as that which is illustrated
in Fig. 4, including the use of spacing pins 36 to hold
the two ~ylinders in a spaced relationship to each
other.
The present invention lies in the provision of a
~ plurality o~ internal vent holes which are formed in
th~ inner cylinder 28 nea.r the upper end thereof. The
vent holes, which are especially sized and placed,
directly connect the sepa:rator body annular moisture
collectlon chamber 30 with the turblne exhaust steam
flow. Thus, this venting arrangement allows the
carrier gas entering the pre-separator wlth ~he
moisture film to f low down into the pre-separator
annular collection chamber 30 thereby substantially
reducing the pressure gradient around the upper
~,~ d~ ~ t''~ ?
~ 14 - 55,1g8
exten~ion cylinder-turbine casing entry ~ap (which
leads to the top of the collection chamber~ and
permittin~ the moisture film on the turbine casing wall
to flow past the en~r~ gap and on in~o the pre~
S separator annular collection chamber.
Other hole sizes and shapes may be ~mployed tha~
could still utilize the concept of internal venting of
a mo~ive fluid, where the motive fluid by conver-ion of
kinetic energy into ~ressure energy generates
sufficient pressure head to directly return the motive
_ fluid into ~he source steam rather than being piped to
lower pressure regions external to the pre-separator
and cross-under piping. Th~ carrier gas, i.e., steam,
is illustrated in Fig. 5 by swirling direc~ional arrows
A which enter the chamber at th~ upper end thereof and
exit through the vent holes 38 As a-result o~ the vent
holes 38, the trapped moisture film will remain
essentially on the outsid~ wall o~ th~ annular
collection chamber, retaining to a large degxee, the
swirling pattern exhibited ln the turb~ne exhaust
casing as the film approaches the pre-separator entry
~ gap (as per Flg~ 1).
In order to as~ure that the annular pre-separator
collectlon chamber 30 ls ~o~itluely pressurized with
respec~ to the cross-under ste~m ~lowlng in the pre-
separator body inner cylinder 28, vent hole sizin~ is
such th~t the individual vent openings are collec~ively
not larger in cross-sectional area than the plane area
defined by the distance between the inner cylinder
outside diameter and the outer cylinder in~ide diameter
- 15 - 55,19
of the pre-separator body multiplied by the vent hole
diameter.
Generally, hole placement is near the upper end of
the inner cylinder as illustrated in Fig. 5. Although
some water droplets will likely pass through the holes,
~hey will be broken up into smaller droplets, which can
be carried within the exhaust steam flow, thereby
eliminated thls moistur~s contributlon to FAC. The
further down the inner cylinder the hol~s are place,
the more droplets wlll likely be carried within ~he
_ exhaust steam flow. Thus, in keeping with the goal o~
mois~ure entrapment, the hole should preferably be
~ept in the upper part of the cylinder.
To furth~r enhance film retention on the outside
wall of the annular collection chamber 30 a plurality
of vanes 40 can be provided radially on the outside
surface of the inn~r cylinder. Additional vanes 42 may
be provided on the inside surface o~ the outer cylinder
26. In one embodiment, vanes 40 and 42 arc used
simultaneously. The vanes 40 and 42 act as channels or
fin~ e members to project radially inwardly into the
collection chamber 30, and are typically perpendicular
to their respect tve mountlng surfaces. The vanes aid
in maintaining, creating, and reinforcing the swirl
pattern o. the two phase flow as the flow proceeds down
into ~he pre-separator annular chamber 30. The vanes
Are generally arranged in a helical screw thread
pattern and act as swirl pattern enhancing means.
While the vanes are illustxated in F~g. 5 as
discontinued segments, they may also be formed as
continuous members, resembling a screw thread. ~ane
~3~ ' q~
- 16 - 55,198
sizing, shape, spacing (pitch between adjacent vanes)
and location may be varied to meet specifie design or
performance requirements. Moreover, patterns other
than screw thread patterns may be used, such as
herringbone patterns, so long as they are capable of
enhancing or creating the swirling pattern. The
swirling pattern provides additional centrifu~al force
which reduces the amoun~ of moisture particles
attaching to the inside sur~ace of the pre-separator
annular collection chamber where the internal vent
holes are located.
Fig. 6 is a cross-sectional view taken along line
VI-VI of Fig. 5. Fig. 6 shows typical cen~er line
locations B of the top row of internal vent holes
provided in the inner cylinder 28 (while the center
lines are shown, the holes themselves have been
omitted). A second, lower row has fewer holes , with
both rows being respectively disposed on opposite sides
of a line which divides the pre-separator ~ody lnto two
cylindrical portlons, one angled with respec~ to the
other. The center lines B illustrat~ typical angular
spacing of the holes which, in Fig. 7 are placed at 22-
1/2 lntervals.
Fig. 7 shows the segmented vanes 40 and 42 pro-
vided on the outer surface of inner cylinder 28 and the
inner surface of outer cylinder 26, respectively. The
vane should extend downwardly fxom the to~ of the pre-
separa~or, but need not extend beyond the vent holes.
Re~erring to Figs~ 8 and 9, the vent holes 38 are
preferably provided with deflectors 44 which are
substantially semiclrcular in shape. The deflectors 44
3 r~) ~
- 17 55,198
partially surround each vent hole opening for the
purpose of reducing re-entrapment o moisture into the
vent holes. When a semicircular deflector is used, the
deflector covers ~he upper portion of the hole since
S induced fluid movemen~ is downward. The de~lectors 44
may be either hole or par~ial circular bosses attached
to the inner diameter of the angular chamber around the
holes or may be inserted pipes threaded or welded into
the vent holes.
The internal venting of the pre-separa~or annular
_ collec~ion chamber 30 provides effective means to
achieve a motive fluid, i.e., steam, to enhance capture
of the water film on the walls of a nuclear turbine
exhaust casing nozzle. Thus, the venting substantially
reduce~, if not eliminates, pressure dlstributions
around the entry gap into the pre-separator sk1mmer~
thereby promoting capture of the water ~ilm on the
turbine casin~ walls. The vent hole~ provide direct
communication of the motive steam between the annular
collection chamber wlth the cross-under pipe and
thereby avoid expansive outside piping. The vented
- ~low is returned int~rnall~y directly into the stre~m
~rom which it was originally moved.
The vanes on either or both of the interior walls
of the pre-separator annular collection chamber enhance
moisture film retention on ~he walls of the ~ollection
chamber and reduce re-entrapmen~ of the moisture
through ~he vent holes~
The structures described increase the moisture
removal efficiency of ilm entrapment pre-separators by
venting an annular moisture chamber to allow
3 ~
- 18 - 55,1g8
communication between the interior of cross~under
piping with the interior o~ the collection chamber. As
a result, large pressure and velocity variations in the
wet steam flow in and around the entry gap ~between the
turbine exhaus~ casing wall and the pre-separator ~
skimmer) are significantly reduced. The removal
effectiveness is further enhanced by providlng swirl-
flow-inducing vanes withln the coll~ction chamber. A
further aspect of ~he methodology is to provide weirs
at the vent holes ~o prevent re-entrapment of captured
_ moisture.
It will be recognized by those of skill in the art
that numerous modifications and adaptations may be made
to the various struçtures and me~hodology disclosed
herein. Thus, it intended by the appended claim~ to
encompass all such modificat~ons which fall wi~hin the
true spirit and scope of the invention.
